This invention relates to processes for forming a protective coating on metal, particularly on: zinc, aluminum, magnesium, and/or zinc, magnesium, and/or aluminum alloy, more particularly aluminum and/or aluminum alloy surfaces. The invention is more particularly related to protective coatings that do not, in contrast to most protective coatings on metals, incorporate any substantial amounts of organic chemical substance. This type of coating is particularly useful for, but is not restricted to, use in heat exchanger surfaces, in which a substantially organic coating layer would impede heat transfer. However, the invention is also applicable to forming a completely inorganic intermediate coating which can then be further coated with other materials, including organic ones such as paint.
Most prior art protective coatings for metals, even in those applications in which heat conduction across the metal surface must be preserved, have required at least one of hexavalent chromium or organic substances to obtain high quality protection. Because of its hazard to workers who come into contact with it and to the general environment, the use of hexavalent chromium is increasingly being economically penalized, or even legally proscribed, in most parts of the world. While most organic substances used in coatings have no such hazardous properties, they do have the disadvantages of often being at least one of expensive, low in heat conductivity, susceptible to damage by heat, and difficult to manage for consistent results in long-continued use when mixed with inorganic materials, as they usually must be to obtain good protection in at least one of the stages of a complete protective coating for metal.
Accordingly, a major object of this invention is to provide a process for forming completely inorganic and hexavalent-chromium-free coatings on metals that will have a protective value at least as good as those now in commercial use for heat exchanger surfaces. Preferably, the coatings provided by the invention will also be at least one of low in cost, easy to manage in long continued use, easily wet by water (i.e., have a low contact angle with water), and high in heat conductivity. Other alternative, concurrent, and/or more detailed objectives will become apparent from the description below.
Except in the claims and the operating examples, or where otherwise expressly indicated, all numerical quantities in this description indicating amounts of material or conditions of reaction and/or use are to be understood as modified by the word “about” in describing the broadest scope of the invention. Practice within the numerical limits stated is generally preferred, however. Also, throughout the description, unless expressly stated to the contrary: percent, “parts of”, and ratio values are by weight or mass; the term “polymer” includes “oligomer”, “copolymer”, “terpolymer” and the like; the description of a group or class of materials as suitable or preferred for a given purpose in connection with the invention implies that mixtures of any two or more of the members of the group or class are equally suitable or preferred; description of constituents in chemical terms refers to the constituents at the time of addition to any combination specified in the description or of generation in situ within the composition by chemical reaction(s) noted in the specification between one or more newly added constituents and one or more constituents already present in the composition when the other constituents are added, and does not preclude unspecified chemical interactions among the constituents of a mixture once mixed; specification of constituents in ionic form additionally implies the presence of sufficient counterions to produce electrical neutrality for the composition as a whole and for any substance added to the composition; any counterions thus implicitly specified preferably are selected from among other constituents explicitly specified in ionic form, to the extent possible; otherwise such counterions may be freely selected, except for avoiding counterions that act adversely to an object of the invention; the word “mole” means “gram mole”, and the word itself and all of its grammatical variations may be used for any chemical species defined by all of the types and numbers of atoms present in it, irrespective of whether the species is ionic, neutral, unstable, hypothetical, or in fact a stable neutral substance with well defined molecules; the term “paint” and all of its grammatical variations include all materials known by more specialized names such as “lacquer”, “varnish”, “shellac”, “primer”, “electropaint”, “top coat”, “color coat”, “clear coat”, “autodeposited coatings”, “radiation curable coatings”, “cross-linkable coatings”, and the like and their corresponding grammatical variations; and the terms “solution”, “soluble”, “homogeneous”, and the like are to be understood as including not only true equilibrium solutions or homogeneity but also dispersions that show no visually detectable tendency toward phase separation over a period of observation of at least 100, or preferably at least 1000, hours during which the material is mechanically undisturbed and the temperature of the material is maintained within the range of 18–25° C.